1. Field of the Invention
This invention relates to a production method for a forged component such as a piston which incorporates a plastic material inside a metallic member.
2. Description of the Prior Art:
Conventionally, components such as pistons for which heat-resistance is required have been produced mainly by heat-resistant aluminum alloy castings or aluminum forgings such as aluminum alloy castings containing nickel (JIS AC8A) and aluminum alloy casings not containing nickel (JIS AC8C). The reduction of the weight of a component of a reciprocating motion system as typified by a piston contributes greatly to the increase in the number of revolutions of an engine, the reduction of striking noise during operation and the improvement in other engine performance.
When the reduction of the weight of components is achieved by use of existing materials such as an aluminum alloy material, rather passive means of analyzing the structure of the component and cutting off its unnecessary thickness is known at present.
If a magnesium alloy casting is employed when the material of the component is changed to other weight-reducing material, the weight becomes about 50% of the weight of the conventional aluminum alloy material and consequently, the effect of weight reduction is great. However, the wall surface of a combustion chamber made of the magnesium alloy must be converted to a heat-resistant structure by use of a ceramic material when the low heat-resistance temperature of magnesium and the level of its high temperature strength are taken into consideration.
If the combustion chamber is made of a plastic material, greater weight reduction can be achieved. However, the heat-resistant temperature of plastic materials is generally about 200.degree. C. or below whereas the temperature of the engine combustion chamber reaches a high temperature of 350.degree. C. and plastic materials cannot withstand this high temperature of 350.degree. C.
Accordingly, in order to accomplish weight reduction of the engine and to obtain a highly heat-resistant structure while using a plastic material, there remains the problem how to manufactures engine components such as pistons.
Japanese Patent Laid-Open No. 119348/1985 discloses a piston for an internal combustion engine and its production method. This piston for the internal combustion engine is constructed whereby the head of the piston is made of a composite comprising a porous inorganic fiber aggregate. A base metal for making the piston is packed into holes in part of the inorganic fiber aggregate and a ceramic material is sprayed onto the surface of the head of the piston so that it can be packed into holes in another part of the inorganic fiber aggregate and a sprayed layer can be formed. Holes in part of the inorganic fiber aggregate are left unpacked by either the base metal or by the ceramic material in such a manner as to form an air layer consisting of porous continuous or discontinuous spaces between the sprayed layer and the base metal.
Furthermore, the production method of a piston for an internal combustion engine disclosed in Japanese Patent Laid-Open No. 119348/1985 described above comprises forming a piston main body by casting while packing a spray metal into holes in part of a porous inorganic fiber aggregate to be fitted to the head of the piston, spraying a ceramic material onto the surface of the head of this piston in such a manner as to be packed into holes in another part of the inorganic fiber aggregate to form a sprayed layer, leaving holes in part of the inorganic fiber aggregate unpacked by the base metal and by the ceramic material, and thus forming an air layer consisting of porous continuous or discontinuous spaces between the sprayed layer and the base metal. When the spray metal is packed into the holes in part of the inorganic fiber aggregate, this production method further includes a step of packing the packing material into the holes in another part of the inorganic fiber aggregate and a step of carrying out the packing process of the spray metal under pressure.